Even in the era of anti-knock additives such as tetraethyl lead, the use of alkylate as a component in motor fuel gained both universal acceptance and importance. In the ensuing years alkylate has become an even more important component of motor fuel. Alkylate is an economical, clean-burning, high-octane, low volatility product that is becoming increasingly important as the composition of gasoline changes in response to environmental concerns and legislation. The governmental regulations most applicable to the increasing importance of alkylates are those affecting lead and butane. Adding lead anti-knock compounds was the easiest way to raise gasoline octane, but because of continuing concerns over the effects of lead emissions the phasing out of lead in gasoline was required, a process over 90% complete. Butane is another effective octane-booster but tends to evaporate from gasoline, especially in warm weather, contributing to smog formation. Recent EPA regulations have effected their virtually complete removal from gasoline.
The term "alkylate" generally refers to a complex mixture resulting from the alkylation of C.sub.2 -C.sub.6 alkenes (olefins) present or formed in a feedstream with intermediates arising primarily from alkanes, especially branched alkanes, and predominantly those with 4 carbon atoms, especially isobutane, also present in the same feedstream. It is most desirable that the complex product mixture from C.sub.4 alkenes and alkanes, referred to as alkylate, contains predominantly trimethylpentanes, since these are high-octane components which add considerable value to motor fuel, yet the chemistry of alkylation affords a dazzling variety of products resulting from only a few basic chemical reactions characteristic of the carbocations, the species which plays a central role in the alkylation process. Thus, chemical processes as chain transfer (intermolecular hydride transfer and alkyl shifts), oligomerization and disproportionation serve to place into the alkylate, as byproduct, materials of from 5-12+ carbon atoms from a feed containing only C.sub.4 alkenes and C.sub.4 alkanes.
The alkylation of alkenes is catalyzed by strong acids generally. Although such alkylation has been the focus of intense and continuing scrutiny for several decades, the requirements of optimum selectivity while achieving high conversion have heretofore narrowed, for all practical purposes, the commercial choice of catalyst to sulfuric acid and liquid hydrogen fluoride. While processes based on each of these acids have gained commercial acceptance those based on HF have been favored at least in part because of the relative ease of HF regeneration. A brief but valuable overview of HF-catalyzed alkylation is presented by B. R. Shah in "Handbook of Petroleum Refining Processes", R. . Meyers, editor, McGraw-Hill Book Company, 1986, pp. 1-through 1-8.
In a rather over-simplified description, the HF-catalyzed alkylation process is carried out as follows. Alkene and isobutane feedstocks are combined and mixed with HF in an alkylation reaction zone. The reactor effluent is separated into the desired alkylate, acid, and other light gases which are predominantly unreacted isobutanes. The HF is either recycled to the reactor directly or regenerated, in whole or in part, prior to its being recycled to the reactor. Unreacted isobutane also is recycled to the reactor, and the alkylate is then used in motor fuel blending.
Recently HF (hydrofluoric acid) has come under environmental pressure. Hydrofluoric acid is classified as an Acutely Hazardous Material, and in Southern California the Board of the South Coast Air Quality Management District recently required that the use of HF in alkylation be phased out by Jan. 1, 1998. Consequently there is increasing reason to seek substitutes for HF as an alkylation catalyst for alkylate production. It is quite desirable to have a solid acid as an effective catalyst, for this permits development of fixed bed processes, a desirable alternative in the petroleum refining industry.
In response to environmental sensitivity to hydrogen fluoride a spate of solid acid catalysts has been suggested as alternative alkylation catalysts, especially Lewis acids such as aluminum halides, boron trifluorides, antimony pentafluoride, and so forth and modified or supported Bronsted acids such as sulfated zirconia. In all processes heretofore suggested the solid acid alkylation catalysts have been used in conjunction with alkenes. Although traditional and conventional alkylation to form alkylate of interest as a motor fuel employs the reaction between alkanes and alkenes, such alkylation processes are encumbered by serious disadvantages when solid acids are used as the catalyst. In particular, the foregoing alkylation process always is accompanied by oligomerization of alkenes, which also is an acid-catalyzed reaction, and the relative amount of oligomerization increases radically when a solid acid is used as the alkylation catalyst relative to the use of, for example, HF as the alkylation catalyst. The traditional alkylation process catalyzed by a solid acid catalyst also is plagued by limited stability of the catalyst; lifetimes under about 6 hours are common.
Thus there are two problems to be overcome in solid bed alkylation process using an alkane-alkene mixture; oligomerization of the alkene and a short catalyst lifetime. One solution to this problem is to conduct alkylation at a very high alkane to alkene ratio, say 100:1. However, such a high ratio is impractical because it requires a very large recycle stream and because of the increased reactor size required for a given productivity (e.g., as measured by alkylate formed per hour). We conjectured that substitution of an alkene by an alkyl halide might provide results equivalent to a very high alkane/alkene ratio. In fact our speculations proved correct. Equally important is our observation that only a partial substitution of an alkene by an alkyl halide also provides substantial benefits. The resulting process of alkylation using an alkane and an alkene-alkyl halide mixture in the presence of a solid acid catalyst is accompanied by substantially lower oligomerization and, even more importantly, substantially increased catalyst stability as measured by catalyst life.
There appears to be little relevant to our discovery in the prior art. U.S. Pat. No. 3,585,252 describes the preparation of alkylate by coupling alkyl halides with organoaluminum compounds, especially trialkylaluminum compounds. In U.S. Pat. No. 4,229,611 the patentee describes the use of a strong Lewis acid system to catalyze the alkylation reaction between alkyl halides and alkenes. The use of clays to convert C.sub.1 -C.sub.4 monohaloalkanes into hydrocarbons of a higher carbon number than the individual reactants is described in U.S. Pat. No. 4,579,996. The clays used contain hydrogen ions and/or metal cations introduced by exchange or deposition, and the use of pillared layered clays is preferred. Finally, GB 545,142 teaches alkylation of, e.g., isobutane with highly oligomerized olefins which are depolymerized by HCl, introduced either per se or via use of an alkyl halide as an activating agent whose sole function is to provide the requisite HCl as a result of its dehydrohalogenation under reaction conditions. Note that the patentee requires the use of an oligomerized alkene, thus perforce fails to recognize the detrimental effect of such oligomers on catalyst life, and also fails to recognize the use of alkyl halides as alkylating agents for the alkylation of alkanes. We note that an explicit requirement of our invention--which is a logical consequence of the detrimental effect of olefin polymers--is that the feedstock be substantially free of polymeric alkenes.
Although the prior art practices alkylation of alkanes with alkenes in the presence of halides, the latter generally are present on the order of 1000 ppm, which corresponds to a molar ratio of halide to olefin of about 0.05. However, no one previously has recognized an incremental benefit resulting from the use of the substantially higher halide levels practiced in this invention.